Femtosecond laser is one of the most powerful new tools developed on the basis of laser science in the past 20 years. Femtosecond pulses are so short that they are within 4 fs presently. One femtosecond (fs) (i.e., 10−15 s) is one quadrillionth of second; if 10 fs is used as a geometric mean to measure the universe, the life of the universe is only 1 min.; femtosecond pulses are so strong that the maximum pulse peak power obtained with a multi-stage chirped pulse amplification (CPA) technique may be at the order of hundreds of terawatts (TW, i.e., 1012 W) or even patawatts (PW, i.e., 1015 W) and their focusable intensity is higher that the energy intensity of all light radiated from the sun to the earth after the light is focused to a point as small as a pinpoint. Femtosecond laser is absolutely a miracle created by human.
In the last 20 years, lasers have been developed from dye lasers to Ti sapphire femtosecond lasers that are mode-locked by means of a Kerr lens, and then to diode pumped all-solid-state femtosecond lasers and femtosecond optical fiber lasers. Notwithstanding that the records of pulse width and energy have been broken continuously, the greatest advancement is that it becomes very easy to obtain ultrafast femtosecond pulses. R. Trebino from Sandia National Laboratories said, “In the past 10 years, the (ultrafast) technique has been improved significantly, and Ti sapphire lasers and today's optical fiber lasers are making the operation of such (femtosecond) lasers simple and stable. Such lasers are commercially available now, but you had to set up them by yourself ten years ago.”
Based on the ultra-short and ultra-strong characteristics of femtosecond laser light, researches in the applied research domain may be generally classified into researches on ultrafast transient phenomena and researches on ultra-strong phenomena. Both types of researches are deepened and developed continuously as the laser pulse width is decreased and the pulse energy is increased. The most direct application of femtosecond pulse lasers is light sources for various time resolved spectroscopy techniques and pumping/probing techniques. The development of femtosecond pulse laser has directly driven the researches in physics, chemistry, biology, material science and information science into the domain of microscopic ultrafast processes, and has opened some fire-new research domains, such as femtosecond chemistry, quantum control chemistry, and coherent spectroscopy of semiconductors, etc. Utilizing femtosecond pulsed laser and nanoscopy in combination, people can explore the carrier dynamics in the nanostructures (quantum wires, quantum dots, and nanocrystals) of semiconductors. In the aspect of biology, people are utilizing differential absorption adsorption spectroscopy and pumping/probing techniques provided on the basis of femtosecond laser technology to explore the energy transfer, energy conversion, and charge separation processes in reaction centers of photosynthesis. Ultra-short pulse laser is further applied for information transmission, processing, and storage.
The first desktop TW laser implemented with a chirped pulse amplification technique successfully started to operate in 1988, marking the researches on femtosecond ultra-strong and ultra-high-density light in laboratories. In the researches in that domain, since the effect of the ultra-short laser field is equivalent to or greatly stronger than the effect of the binding field suffered by the electrons in atoms, the perturbation theory is not true anymore, and a new theory has to be developed. At the order of 1020 W/cm2 light intensity, researches on simulated astrophysical phenomena can be made. Thermoelectrons (200 keV<E<1 MEV) produced under ultra-strong laser light at 1019-1021 W/cm2 light intensity can heat up a large quantity of ions and thereby initiate nuclear fusion. The final implementation of the concept of fast ignition for inertial confined fusion (ICF) will make inestimable contribution to national security and energy utilization.
Another important application of femtosecond lasers is micro-fine processing. Generally, according to the laser pulse standard, laser pulses with duration longer than 10 picoseconds (equivalent to the heat conduction time) belong to long pulses. If such laser pulses are used to process materials, the processing accuracy will be degraded because the thermal effect causes changes of the adjacent material. In contrast, femtosecond laser pulses, which have pulse width as small as one trillionth second, have unique material processing characteristics, for example, the fused area of a processed hole is very small or even doesn't exist; micro-machining or micro-engraving in a variety of materials, such as metal materials, semiconductor materials, transparent materials, or even biological tissues, etc., can be realized; the processed area may be smaller than the focus size, and the diffraction limit can be breached, etc. Some automobile manufacturers and heavy equipment fabricators are making research on how to utilize femtosecond laser to process fuel injection nozzles of engines better. Pinholes in width as small as hundreds of nanometers can be formed in metal materials with ultra-short pulse laser light. In the meeting of Optical Society of America (OSA) held in Orlando lately, Hayter from IBM Corporation said that IBM had applied a femtosecond laser system in the photolithographic process of large scale integrated circuit (LSIC) chips. There is little or no heat transfer when femtosecond laser light is used for cutting. Researchers of Lawrence Livermore National Laboratory (LLNL) found that such laser beams could be used to cut high explosives safely. Laszk from LLNL said, “Femtosecond laser is expected to be a cold processing tool to defuse decommissioned rockets, artillery shells, and other weapons.” Femtosecond laser light can be used to cut fragile polymeric materials without changing important biochemical characteristics of the polymeric materials. Biomedical experts have used femtosecond laser light as an ultra-precision surgical knife for vision correction operations. Utilizing femtosecond laser light as a surgical knife can reduce tissue damages and void postoperative sequelae, and even allows precision operation to a single cell or gene therapy. Presently, people are make research on how to apply femtosecond laser in dental treatments. Some scientists have found that a small part of a tooth can be removed with ultrashort pulse laser light without affecting the peripheral substances. It is believed that femtosecond laser will be applied more widely in more domains as ultra-short pulse laser techniques are developed further and high-reliability commercial femtosecond lasers are further improved.
At present, lasers that are used the most commonly are Ti-sapphire femtosecond lasers. The main principle of those lasers is the self-mode-locking effect of Ti-doped sapphire. The self-mode-locking phenomenon of Ti-doped sapphire laser was found by Spence et al. in Scotland in 1990. The emergence of that technique opened a fire-new page of research on ultra-short pulse laser. Different from the traditional active mode locking and passive mode-locking, for some laser oscillators that contain a medium with strong Kerr effect, with a specific oscillator structure, stable mode-locked operation can be realized without any additional modulation or saturable absorber. The combination of such a simple structure and Ti-doped sapphire laser with ultra-wide tunable bandwidth has become a main trend of development of ultra-short pulse lasers now, and has directly produced light pulses shorter than 5 fs. At present, it is universally accepted that the basic principle of the mode locking technique lies in the Kerr effect formed by a solid gain medium under strong focal pumping. It is well-known that the refractivity of a medium as a result of the Kerr effect under the action of nonhomogeneous light may be expressed by the following formula:n(r)=n0+n2I(r)
Where, no is static refractivity independent of light intensity, n2 is Kerr coefficient, I(r) is light intensity distribution. Thus, under the action of pump light and oscillating light, the distribution of refractivity of the medium will change in the radial direction, and the gain medium will be equivalent to a self-focusing lens. When the oscillating laser light passes through the medium, stronger light and weaker light exhibit two different focusing modes, and they exhibit different light beam sizes at different places in the resonator. On that basis, if a hard edge diaphragm in appropriate size is added at a place in the resonator in a way that the transmitted light beam resulted from strong focusing of the stronger light right passes through the diaphragm while the transmitted light beam resulted from weak focusing of the weaker light can't pass through the diaphragm, the system will be equivalent to a fast saturable absorber, and a stable self-mode-locking process will be established when the laser light reaches dynamic balance after multiple times of to-and-fro oscillation. The result of further research has proved: with a certain oscillator structure, relative distribution of pump light-oscillating light similar to the case of a diaphragm also exists in laser crystals that have a self-focusing effect. Such a mechanism usually is also referred to as soft edge diaphragm. With the soft edge diaphragm technique, the structure of a self-mode-locking laser is simpler, and the mode locking adjustment is more convenient.
At present, all common mode locking techniques are implemented in the oscillator, by inserting an acoustooptic modulator in the oscillator, or pushing lenses or prisms in the oscillator, or inserting a diaphragm in the oscillator. Since all of the mode locking techniques are implemented in the resonator regardless of the specific implementation scheme, they may cause instability of the resonator or introduce loss or dispersion into the resonator, and consequently cause degraded stability of the pulse width or output of the femtosecond laser light. In view of the above problems, the present invention provides a femtosecond laser, which utilizes an innovative mode locking device to improve the stability of femtosecond laser and shorten pulse width.